Light pole structures are designed to accommodate certain specific environmental, load and aesthetic requirements. Light poles, traffic mast arms and similar structures are subjected to naturally-induced vibrations which cause damages/failures of such structures. Probably the most common vibration problems are created by wind flow over the pole and also wind gusts on facing surfaces which causes vibration of the poles. Traditional lightly-damped structures are flexible and highly susceptible to wind-induced vibration.
Wind gusts can result in excessive vibration. For cantilever mast arm structures this can result in horizontal out-of-plane motion perpendicular to the length of the mast arm. This kind of motion along the length of the mast arm is minimal, resulting in motion predominately in a single direction.
A number of different methods and apparatuses have been suggested to reduce the excessive wind-induced vibration of the poles. One such apparatus includes a pole damping system in which a hollow tubular member is attached to the wall of a hollow pole with an inertia mass in the form of a solid rod on the interior of the tube, which is unattached to the tube for limiting movement on the interior of the tube for damping vibrations of the pole. In another apparatus, the tubular member is mounted on the exterior of the pole. For each of these types of systems, the vibration dampening occurs only at the midpoint of the pole and mainly for a second mode of vibration. Accordingly, these systems are ineffective at dampening vibrations to a broad range of excitation intensities and when operational can result in loud noises.
Additional prior art further includes a pole vibration damping system having an annular housing with an internal radial pole encircling chambers. In this system, lead spheres are provided for effecting damping of first mode harmonic vibrations with the assembly being attached to the upper end portion of the pole and operating in conjunction with a pole vibration damping device mounted medially for damping second harmonic mode vibrations. The downside to this alternative system is that it provides a flat floor that is only effective when the amplitude is large enough that the moving mass hits the walls of the pole vibration damping device and the energy dissipation is achieved through repeated impacts. Such pole vibration damping systems do not provide effective dampening for small amplitudes and can result in loud noises.
Based on the foregoing, there is a need for a method and apparatus for providing effective damping of various modes of vibrations for a range of different types of poles. Such a needed device would provide effective dampening to a broad range of excitation intensities. The device would be effective for both small and large amplitudes and act as a vibration absorber. Further, the method and apparatus would dissipate energy through friction of damping weights, pneumatic damping, viscous damping and/or through eddy current dampening. Finally, the method and apparatus would be relatively quiet in operation. The present invention overcomes prior art shortcomings by accomplishing these critical objectives.